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Most common mistakes on first advanced combustion cannon?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: chubbylad13 » Fri Nov 05, 2010 10:32 pm

Ok, Iamb math isn't my strongest subject, so If my barrel is 4" of 2.5" pvc what lenght of 4" pvc would I use for the chamber. when I calculated it the volume of the barrel came out to be 19.56. 80% of that is 15.64 so does that mean the chamber should be 8 inches? I know I am an Idiot but It would be great if someone could tell me what I did wrong. :?
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Unread postAuthor: chubbylad13 » Sat Nov 06, 2010 2:50 pm

Technician1002 wrote:I would recommend doing the math. Burnt lake has shown that a chamber barrel ratio of about 0.8:1 works well.

Decide on a barrel size say 4 feet to keep the full size reasonable. An 8 to 9 foot overall length is a little awkward for most people. A 5 foot barrel with a 4 foot chamber is way too big.

Find the volume of the 2.5 inch barrel and then calculate the size of chamber to match it with a volume of about 80% of the barrel.

In volume a 4 foot long 4 inch chamber is way too big for a 5 foot 2.5 inch barrel.

Area times length = volume of a cylinder.
Area = Pi X Radius squared.

For example a 4 foot 4 inch chamber would have a cross section of;
3.14 X 2 squared. 3.14 X 4 = 12.56 square inches.

Volume is area X length.
12.56 Sq in times 48 inches (use the same units of measure) = 602.88 Cubic inches.

Do the math for your proposed sizes.


Ok, I did the math and I think its correct this time.For the chamber, The equation said I should use 14.94 inches of 4", if my barrel is 48 inches of 2.5". Does this seem correct to you?
Thanks :D
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Unread postAuthor: willarddaniels » Sat Nov 06, 2010 10:56 pm

chubbylad13 wrote:Ok, I did the math and I think its correct this time.For the chamber, The equation said I should use 14.94 inches of 4", if my barrel is 48 inches of 2.5". Does this seem correct to you?

Those numbers would give you a chamber/barrel ratio of 0.8 (roughly)

If you are creating an over/under, remember that part of your chamber is in the elbows. Use a 12" long 4" section of pipe for your main combustion chamber. After you assemble the rest of it (elbows, reducer), measure the volume with water. Use this figure as your starting point to see what your optimal barrel length will be.
You don't have to have everything glued together to gauge this, for you may consider adjusting the chamber further if you find you need a 7' barrel for optimization.
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Unread postAuthor: Fnord » Sun Nov 07, 2010 2:02 pm

^ Well, you've been away for a bit :)


I think the generally accepted view is a ~0.8:1 ratio will give you the most power for a specific chamber size, and ~1.2:1 will give the most power for a specific barrel size. Though the former will be quiet and the later loud.
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